1. Field of Invention
The present invention pertains to the field of information storage units. More particularly, this invention relates to an information storage unit using an array of light beam emitters or near-field optical sources to write and read data in several novel information storage media.
2. Background
Electronic devices, such as palm computers, digital cameras and cellular telephones, are becoming more compact and miniature, even as they incorporate more sophisticated data processing and storage circuitry. Moreover, types of digital communication other than text are becoming much more common, such as video, audio and graphics, requiring massive amounts of data to convey the complex information inherent therein. These developments have created an enormous demand for new storage technologies that are capable of handling more complex data at a lower cost and in a much more compact package.
One response to this demand has been the development of ultra-high density storage devices, such as the one described in U.S. Pat. No. 5,557,596 granted to Gibson et al. on Sep. 17, 1996. This system provides for a plurality of electron emitters generating beams of electrons to information storage media areas on a movable platform to store and retrieve information. A micro mover, based on micro electro mechanical systems (MEMS) technology moves the platform relative to the electron emitters to enable parallel communications with selected storage media areas on the platform. In the Gibson et al patent, an electron beam impacts storage media areas at different intensities, selectively altering some aspects of the storage material, such as changing the state of the storage material between amorphous and crystalline phases or between different crystalline phases that affects the response of, for example, a diode storage medium to a readback electron beam.
There is a continued need for increased miniaturization and expanded ability to handle greater quantities of more complex data at a faster speed and in even more compact areas. Efforts are now underway to adapt technology disclosed in the Gibson et al patent to enable the storage of data on a scale of ten nanometers (100 angstroms) up to hundreds of nanometers.
Several challenges arise in attempting to store data at this level. The processes of information storage and retrieval become increasingly difficult tasks, and writing data with electron beams presents several limitations. It is possible to use low energy electrons in this technique to avoid problems with dielectric breakdown, field emission from undesirable locations, and the need for relatively large and expensive power supplies. However, low energy electrons have very short penetration depths, making this approach highly susceptible to the surface conditions of the medium. Moreover, only very thin layers may be present on the top of the storage media, making difficult the use of a protective layer or a conducting electrode on top of the storage layer. In addition, the stability and cyclability of a storage device using electron-readback may be limited by the mechanical and thermal properties of the free surface of the storage medium. Only very thin protective cladding layers can be used with a low-energy electron-beam addressing scheme, as these layers would prevent access by low energy electrons.
In some miniature storage devices, such as CD-RW and DVD-RW drives, data is written using optical devices, such as lasers, to reversibly change the optical reflectivity of a storage medium. The diffraction-limited spot size of the lasers sets a lower bound to the size of bits to be written. Attempts have been made to circumvent the diffraction limit by using near-field light sources that provide evanescent light emitted through a small aperture. Typically, in this approach, light from a laser is emitted through an aperture having a diameter less than the wavelength of the light. Alternately, the laser is coupled with a fiber optic cable tapering down to a diameter smaller than the light wavelength and coated with a metal. In either case, an evanescent field protrudes from the aperture a short distance, resulting in the transfer of energy (evanescent coupling) with a storage medium disposed at a distance of less than a wavelength of the light from the aperture (near-field). See U.S. Pat. No. 6,185,051 granted to Chen et al. on Feb. 6, 2001. In some cases, evanescent coupling can produce a very high-quality effect, inducing more than 50% of the energy in the source of radiation to couple with the material in the near field. It is also possible to implement the present invention utilizing apertureless near-field sources, such as those described in F. Zenhausem, M. P. O'Boyle, and H. K. Wickramasinghe. Apertureless near-field optical microscope. Applied Physics Letters, 65(13): 1623–1625,1994.
Although the near-field optical method is promising for writing data at substantially increased densities, this approach presents problems in the task of reading the data. Sensing of the data may be achieved by light reflected back into the laser cavity through the small aperture. This reflected light causes a change in the output of the laser that can be monitored to detect changes in reflectivity and, thereby, the presence of bits. However, the amount of reflected light is so small that there is only a very small change in the output power of the laser, making detection difficult and susceptible to error.
To read miniature data bits, it is desirable to use a technique capable of producing substantially larger signals. Thus, structure and methods are needed to store and read high density data such that detection of data is more readily obtained.